Add Arbitrum bridge docs

Author: krebernisak

CHANGELOG.md

@@ -9,6 +9,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 ### Added
 
 - Access controlled mintable & burnable LinkToken, for use on sidechains and L2 networks.
+- Add Arbitrum bridge documentation
 - Added versioning to v0.6 & v0.7 contracts
 
 ### Changed

contracts/v0.7/bridge/README.md

@@ -23,3 +23,5 @@ NOTICE: Current implementation of LinkTokenChild contract requires some addition
   gets defined, and once online transfer the ownership (bridge gateway role) to the new bridge.
 
 TODO: Potentially create an upgradeable `LinkTokenChild.sol` and limit gateway support to only one (owner)!
+
+- `./token/arbitrum/`: Documentation for the Arbitrum bridge.

contracts/v0.7/bridge/token/arbitrum/README.md

@@ -0,0 +1,8 @@
+# LINK Token on Arbitrum
+
+Arbitrum included the ERC677 protocol support into their standard bridge token implementation, so **custom integration is not necessary**.
+
+- Arbitrum ERC20/ERC677 token implementation: [StandardArbERC20.sol](https://github.com/OffchainLabs/arbitrum/blob/042bcafbccf09a23c149822436fb5ec0f4f6fe57/packages/arb-bridge-peripherals/contracts/tokenbridge/arbitrum/StandardArbERC20.sol)
+- _Ethereum Rinkeby Arbitrum_ LINK token address: [0x615fBe6372676474d9e6933d310469c9b68e9726](https://rinkeby-explorer.arbitrum.io/address/0x615fBe6372676474d9e6933d310469c9b68e9726)
+- _Ethereum Mainnet Arbitrum One_ LINK token address: [0xf97f4df75117a78c1A5a0DBb814Af92458539FB4](https://explorer.arbitrum.io/address/0xf97f4df75117a78c1A5a0DBb814Af92458539FB4)
+- Arbitrum bridge UI: [https://bridge.arbitrum.io/](https://bridge.arbitrum.io/)